1. Field of the Invention
The invention relates to a gas distributor for a Siemens reactor for deposition of polycrystalline silicon, in particular a feed gas distributor and an off-gas collector.
2. Description of the Related Art
High-purity polycrystalline silicon (polysilicon) serves as a starting material for producing monocrystalline silicon for semiconductors by the Czochralski (CZ)—or zone melting (FZ)—method, and also for producing monocrystalline or multicrystalline silicon by various drawing and casting methods for producing solar cells for photovoltaics.
Polysilicon is usually produced by means of the Siemens process. In this process, a reaction gas comprising one or more silicon-containing components and optionally hydrogen is introduced into a reactor comprising substrates heated by direct passage of current, wherein silicon deposits in solid form onto the substrates. The silicon-containing components preferably used are silane (SiH4), monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) or mixtures of these substances.
The Siemens process is usually carried out in a deposition reactor (also termed “Siemens reactor”). In the most common embodiment, the reactor comprises a metallic baseplate and a coolable bell which is seated on the baseplate in such a manner that a reaction space is formed in the interior of the bell. The baseplate is furnished with one or more gas inlet openings and one or more off-gas openings for the departing reaction gases, and also with holders with which the substrates are held within the reaction space and are supplied with electric power.
Each substrate consists mostly of two thin filament rods and a bridge which generally joins the adjacent rods at their free ends. Most frequently, the filament rods are made of monocrystalline or polycrystalline silicon; more rarely, metals or alloys or carbon are used. The filament rods plug vertically into the electrodes situated on the reactor base, via which electrodes the connection to the power supply is made. On the heated filament rods and the horizontal bridge, high-purity polysilicon deposits, as a result of which the diameter thereof increases with time. After the desired diameter is achieved, the process is terminated.
To produce polysilicon, routinely deposition reactors are used in which, in the lower part, what is termed the baseplate, the reaction gas is injected via nozzles. An introduction of the reaction gas in the upper part of the reactor by means of nozzles is, however, equally possible.
Likewise, off-gas that is formed is frequently removed from the reactor via one or more openings in the reactor base, but also via the reactor hood.
Since a uniform distribution of the feed gas is important for the uniform deposition on the rods, the gas is usually fed via a plurality of nozzles.
Such a distribution of the feed gas can be effected either by means of a multiplicity of individual feed gas lines each having direct connection to the individual feed gas nozzles, or via a gas distributor, e.g. a ring-type or a different type of gas distributor in the vicinity of the reactor usually beneath the reactor baseplate, having a plurality of connections for the individual feed gas nozzles.
In the prior art, some corresponding solutions are known.
US2011/0058988 A1 describes a reactor for producing polycrystalline silicon, having a reactor base which has a multiplicity of nozzles each having one nozzle entry opening and each having a nozzle outlet opening, which form a feed for a silicon-containing gas into the reactor interior, characterized in that one wall is shaped in such a manner that, together with an outer surface of the reactor base, it delimits a cavity which forms a distribution of the silicon-containing gas onto at least one part of the nozzles with which it communicates, wherein the wall is mounted gas-tightly on the reactor base in such a manner that at least one contact surface of the cavity with the outer surface of the reactor base is restricted to a true subregion of the outer surface of the reactor base.
US2011/0058988 A1 has the object of providing a reactor for producing polycrystalline silicon, the reactor base of which is constructed in such a manner that the distribution of the silicon-containing gas to the nozzles in the reactor base is space-saving, reliable and inexpensive, and a ready accessibility to the further devices on the outside of the reactor base, e.g. electrodes or coolant connections, is permitted.
US2011/0305604 A1 describes a reactor for producing polycrystalline silicon by the monosilane process, having a reactor base which a multiplicity of nozzles has formed, through which a silicon-containing gas flows in, a plurality of filament rods likewise mounted on the reactor base and a gas outlet opening at a distance from the nozzles for feeding used monosilane to an enrichment and/or reprocessing, characterized in that the gas outlet opening is formed on a free end of an inner tube and in that the inner tube is guided through the reactor base, and in that the inner tube possesses an outer wall and an inner wall and thereby forms an intermediate space in which at least one cooling water circuit is guided. US2011/0305604 A1 has the object of designing a reactor for producing polycrystalline silicon in such a manner that the removal of used silicon-containing gas (monosilane: SiH4) is designed in such a manner that reliable production of the polycrystalline silicon is ensured.
In a preferred embodiment, the reactor and the reactor base are constructed as a jacket comprising an inner wall and an outer wall, wherein the water for cooling is situated in the jacket. In this case, the reactor base comprises a first region and a second region, wherein the first region is formed by a reactor interior-facing plate of an intermediate plate which carries the nozzles, and wherein the second region is formed by the intermediate plate and a baseplate which carries the supply connections for the filaments.
The reactor has a multiplicity of nozzles for feeding the reaction gas. Via an intermediate plate, the reactor base is subdivided into two regions. The nozzles are constructed as a cylindrical connection between the intermediate plate and the interior of the reactor. Therefore, the region beneath the intermediate plate functions as a gas distributor.
U.S. Pat. No. 4,805,556 A describes a reactor for deposition of polycrystalline silicon by pyrolysis of a silane, which provides reprocessing off-gas from the reactor and at least in part returning it to the reactor. For this purpose, a ring-type distributor grid of a distributor piece and a multiplicity of distributor rings which correspond in number to the filaments are provided.
In a preferred embodiment, the distributor grid comprises two distributor pieces, wherein a first distributor piece is connected to a tube for reprocessed off-gas, and the second distributor piece is connected to the distributor rings, wherein the two distributor pieces are connected by means of at least one feed line. Each distributor ring comprises a multiplicity of nozzle openings which are arranged symmetrically. The distributor rings are placed in each case in such a manner that the reprocessed off-gas is directed towards a filament-carrying electrode.
Because the feed gases and off-gases are moisture-sensitive, and in the event of mixing with air or oxygen, formation of explosive gas mixtures can occur, feed gas lines and off-gas lines or distributors are firmly welded to the reactor base, cf. US2011/0058988 A1. This prevents escape of the reaction gas and simplifies certain maintenance work (e.g. no monitoring and no replacement of seals is necessary).
Through the deposition process, crack formation can occur in the growing silicon rods. As a consequence thereof, silicon fragments of various size can fall into feed openings and off-gas openings and block the gas lines or gas distributors.
In the worst case, all rods can disintegrate, wherein very large silicon pieces can fall into the inlet and outlet openings.
In the case of the feed gas, such a blockage leads to an uneven distribution or incorrect metering of the feed gas, and in the case of the off-gas, blockage can cause an undesirable pressure rise in the reactor.
For this reason, the silicon pieces must be removed at the latest before the following batch.
In the case of a gas distributor which is firmly connected to the reactor base, i.e. which can only be separated from the reactor base for example via cutting, in practice there is only the possibility of removing the silicon pieces in the batch change via the gas openings in the reactor.
Silicon pieces may usually be removed only with substantial effort (e.g. by hand or using a gripping tool or vacuum cleaner). This procedure is laborious and time consuming (i.e. relatively long set-up times) and also does not always lead to success, since very large pieces are sometimes too bulky for the gripping tool and cannot be removed by suction using the vacuum cleaner.
In a gas distributor or off-gas collector which is fastened to the reactor with the aid of detachable connections (e.g. a flange) via the feed-gas or off-gas outflow lines, although the feed-gas or off-gas lines can in fact be separated, the gas distributor or off-gas collector per se, in which most of the silicon pieces are found, cannot be opened. Additional piping and electrical connections beneath the reactor base, moreover, prevent simple access to the gas distributor and/or off-gas collector.
From these problems, there resulted the objective of the invention.